The present application is based on and claims priority from Japanese Patent Application Hei 11-137019 filed May 18, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a DC motor-assisted power steering apparatus.
2. Description of the Related Art
A power assisting DC motor (hereinafter referred to as P.A. motor) of an ordinary motor-assisted power steering apparatus is driven by a motor drive unit which has a H-bridge circuit, a current detecting circuit, and current control means. The H-bridge circuit has four power transistors to supply the DC motor with pulse-width-modulated-drive-current. The current detecting means detects drive current of the DC motor. The current control means controls the H-bridge circuit in a PID (proportional integral derivative) control on the basis of a difference between a command current value and an actual drive current.
The command current value is provided according to a steering torque signal and a vehicle speed signal. An ordinary current control means controls a motor drive circuit in the manner of pulse width modulation (hereinafter referred to as PWM-control) on the basis of the command current value and the detected actual current value of the P.A. motor. In the current control means, PI (proportional integral) feedback control is carried out by the PI control operation means on the basis of the current difference between the command current value and the actual current value to determine a duty ratio. In addition, a direction command value is provided to indicate the direction of the command current value.
However, if the PI feedback control is repeated according to an integration formula having an integral compensation term, the integral compensation term may increase to a value that cannot be neglected due to accumulation. As a result, a large impulsive drive current may flow in the P.A. motor, thereby causing a large impulsive torque or vibration applied on the steering wheel.
Therefore, driving feels unnatural due to unpleasant bumps on the steering wheel.
JP-A 10-203384 discloses a motor-assisted power steering apparatus which basically carries out a digital feedback control on the basis of a difference between command current value and actual current value. If an actual current value is excessively large, it is limited to a predetermined value, while an internal parameter is changed, so that the response characteristic of the steering can be improved.
Although the duty ratio is limited when power assisting motor (P.A. motor) is reversed, a considerable amount of the accumulated portion corresponding to the integral term still remains. As a result, abnormal-current detecting means may be erroneously operated, or the DC motor may suddenly vibrate due to an excessive amount of impulsive current flows in the P.A. motor when it is reversed. This results in bumps on the steering wheel during the reversal of steering operation, thereby giving the driver an unpleasant feeling.
Therefore, the present invention has an object of providing a P.A. motor drive unit in which an impulse drive current, shocking motion, or vibration is restrained even when the P.A. motor is reversed.
Another object of the invention is to provide a motor-assisted power steering apparatus adopting the P.A. motor drive unit.
According to a feature of the invention, a P.A. motor drive unit for driving a P.A. motor according to a command current value includes an H-bridge circuit for supplying PWM-controlled drive current, current detecting means for providing an actual current value, and current control means for carrying out a PI feed back control according to difference between the absolute values of the command current value and the actual current value. In the above motor drive unit, the current control means is comprised of PI operation control means for providing a PI control equation having an integral compensation term, and compensation term operation means for resetting the integral compensation term to a suitable value when operation of the P.A. motor is changed from one state to another.
Even if the integral compensation term increases when the P.A. motor is reversed or changed to operate in a different state, integral compensation term is forced to reset to a value close to zero or a negative value. Accordingly, a large impulsive drive current may not be generated since the duty ratio of the drive current is controlled to be small. As a result, the response characteristic of the drive current to the command current value is improved, and the P.A. motor can operate smoothly. Since an impulsive large drive current is restrained, an erroneous operation of a fail-safe means can be prevented when detecting abnormal current. Moreover, since an impulsive current change is restrained, EMI trouble due to radiation of noises to the surrounding devices can be restrained.
In the P.A. motor drive unit as defined above the change of operation of the P.A. motor is judged by a change of the symbol of the command current value.
In the above defined P.A. motor drive unit, the compensation term is reset when the symbol of the command current value is changed and the direction of the command current value becomes different from the rotation direction of P.A. motor.
If the integral compensation term is reset only at the state of operation of the P.A. motor when such excessive accumulation may very likely occur, linearity of the control system will be maintained.
In the P.A. motor drive unit as defined above, the compensation term is reset when the P.A. motor is started or when it is restarted after an emergency stop due to an abnormal operation.
According to another feature of the invention, a power steering control apparatus for a vehicle is comprised of command current operation means for providing a command current value according to a steering torque applied to a steering shaft and a vehicle speed, the P.A. motor drive unit defined above, and a P.A. motor.
If the operation of the P.A. motor is changed, bump is prevented from being applied to the steering wheel. As a result, the feeling of the steering vehicle is improved. Because an abrupt change of the impulsive drive current is restrained, surrounding devices are not negatively affected by radiation of the noises, including EMI.